UEFI does not pass kernel parameters to the kernel during normal boot, so you need to hardcode them via CONFIG_CMDLINE. Example for the root partition on /dev/sda2:
[Collapse] Kernel configurationEnable built-in kernel parameters
Processor type and features --->
[*] Built-in kernel command line
(root=/dev/sda2)

My ZFS is rpool/ROOT/roots. What would I enter for the Built-in kernel command line?